The present invention is in the general field of biological receptors and the various uses that can be made of such receptors. More specifically, the invention relates to nucleic acids encoding a novel galanin receptor and the receptor protein itself.
Galanin is a small (29-30 amino acid) neuroendocrine peptide which does not belong to any known peptide family (Bedecs et al., Int. J. Biochem. Cell. Biol. 27: 337-349 (1995)); It is widely distributed in the central nervous system and other tissues, and has been reported to have a large number of diverse biological and pharmacological activities. Galanin has been reported to: (a) promote growth hormone release (Bauer et al., The Lancet 2:192-195 (1986)); (b) inhibit glucose-induced insulin release (Ahren et al., FEBS Lett. 299:233-237 (1988)); (c) regulate motility in the gastrointestinal tract (Fox-Thelkeld et al., Gastroenter-ology 101:1471-1476 (1991)); (d) stimulate feeding behavior (Crawley et al., J. Neurosci 10:3695-3700 (1990)); and (e) impair cognitive function (Mastropaolo et al., Proc. Nat""l Acad. Sci. U.S.A. 85:9841-9845 (1988)).
Of particular pharmacological interest are galanin""s analgesic effects (Post et al., Acta Physiol. Scand. 132:583-584 (1988)). In the spinal cord, galanin inhibits nociceptive reflexes and potentiates the analgesic effect of morphine (Wiesenfeld-Hallin et al., Neurosci. Lett. 105:149-154 (1989)). Target administration of galanin hyperpolarizes dorsal horn neurons and chronic administration of a galanin receptor antagonist after axotomy has been reported to markedly increase autonomy in rats (Verge et al., Neurosci. Lett. 149:193-197 (1993)). These observations indicate that galanin, like morphine, has strong anti-nociceptive actions in vivo. Thus, the known pharmacological effects of galanin suggest potential therapeutic applications as an anesthetic or analgesic in animals and humans.
Galanin exerts its effects by binding to membrane-bound receptors. The cDNA for one such receptor (xe2x80x9cGAL-R1xe2x80x9d) has been cloned from both humans and rats (Habert-Ortoliet et al., Proc. Natl. Acad. Sci. U.S.A. 91:9780-9783 (1994); Burgevin et al., J. Mol. Neurosci. 6:33-41 (1995)). High levels of rat GAL-R1 mRNA have been found in the ventral hippocampus, thalamus, amygdala, and medulla oblongata of the brain and in the dorsal horn of the spinal cord (Burgevin et al., supra). Pharmacological data obtained using galanin fragments, agonists and antagonists have suggested that more than one type of receptor may be responsible for galanin""s actions (for a review, see Valkna et al., Neurosci. Lett. 187:75-78 (1995)). The isolation and characterization of new receptors for galanin would be highly desirable to assist in the discovery and development of therapeutic agents for altering galanin activity in vivo.
The present invention is based upon the discovery of a novel galanin receptor (xe2x80x9cGAL-R2xe2x80x9d) which is distinct from previously reported receptors in terms of structure, tissue distribution and binding characteristics. Receptors from both the rat and human have been isolated and sequenced. As used herein, the term xe2x80x9cGAL-R2xe2x80x9d refers to the receptor from either of these species unless the text, expressly or by context, indicates otherwise.
In its first aspect, the invention is directed to proteins, except as existing in nature, comprising the amino acid sequence consisting functionally of rat GAL-R2 (as shown in FIG. 1) or consisting functionally of human GAL-R2 (as shown in FIG. 2). The term xe2x80x9cconsisting functionally ofxe2x80x9d refers to proteins in which the sequence of FIG. 1 or FIG. 2 has undergone additions, deletions or substitutions which do not substantially alter the functional characteristics of the receptor. Thus, the invention encompasses proteins having exactly the same amino acid sequence as shown in the figures, as well as proteins with differences that are not substantial as evidenced by their retaining the basic, qualitative ligand binding properties of GAL-R2. The invention further encompasses substantially pure proteins consisting essentially of a GAL-R2 amino acid sequence, antibodies that bind specifically to GAL-R2 (i.e. that have at least a 100 fold greater affinity for GAL-R2 than any other protein), and antibodies made by a process involving the injection of pharmaceutically acceptable preparations of such proteins into an animal capable of antibody production. In a preferred embodiment, monoclonal antibody to GAL-R2 is produced by injecting the pharmaceutically acceptable preparation of GAL-R2 into a mouse and then fusing mouse spleen cells with myeloma cells.
The invention is also directed to a substantially pure polynucleotide encoding a protein comprising the amino acid sequence consisting functionally of the sequence of rat GAL-R2 (as shown in FIG. 1) or human GAL-R2 (as shown in FIG. 2). This aspect of the invention encompasses polynucleotides encoding proteins consisting essentially of the amino acid sequences of in the figures, expression vectors comprising such polynucleotides, and host cells transformed with such vectors. Also included is the recombinant rat and human GAL-R2 proteins produced by host cells made in this manner. Preferably, the polynucleotide encoding rat GAL-R2 has the nucleotide sequence shown in FIG. 1 and the polynucleotide encoding human GAL-R2 has the nucleotide sequence shown in FIG. 2. It is also preferred that the vectors and host cells used for the expression of GAL-R2 use these particular polynucleotides.
In another aspect, the present invention is directed to a method for assaying a test compound for its ability to bind to GAL-R2. This method is performed by incubating a source of GAL-R2 with a ligand known to bind to the receptor and with the test compound. The source of GAL-R2 should be substantially free of other types of galanin receptors, i.e. greater than 90% of the galanin receptors present should correspond to GAL-R2. Upon completion of incubation, the ability of the test compound to bind to GAL-R2 is determined by the extent to which ligand binding has been displaced. A preferred source of GAL-R2 for use in the assay is a cell transformed with a vector for expressing the receptor and comprising a polynucleotide encoding a protein consisting essentially of the amino acid sequence shown in FIG. 1 (rat GAL-R2) and FIG. 2 (human GAL-R2). Instead of using cells in the assay, a membrane preparation can be prepared from the cells and this can be used as the source of GAL-R2. Although not essential, the assay can be accompanied by the determination of the activation of a second messenger pathway such as the adenyl cyclase pathway. This should help to determine whether a compound that binds to GAL-R2 is acting as an agonist or antagonist to galanin.
In another aspect, the present invention is directed to a method for assaying a test compound for its ability to alter the expression of GAL-R2. This method is performed by growing cells expressing GAL-R2, but substantially free of other galanin receptors, in the presence of the test compound. Cells are then collected and the expression of GAL-R2 is compared with expression in control cells grown under essentially identical conditions but in the absence of the test compound. In preferred embodiments, the cells expressing GAL-R2 are cells transformed with an expression vector comprising a polynucleotide sequence encoding a protein consisting essentially of the amino acid sequence shown in FIG. 1 (rat GAL-R2) or FIG. 2 (human GAL-R2). A preferred test compound is an oligonucleotide at least 15 nucleotides in length and comprising a sequence complimentary to a sequence shown in one or both of the figures. The preferred method for determining receptor expression is by means of a receptor binding assay.